The field of the invention is coordinate measuring styli.
A coordinate measuring machine (CMM) (e.g. a Zeiss Prismo VAST) is generally used for precise measurement of the shape and size of parts. Traditional CMMs operate by recording information about points along the surface of the part being measured. The information may be used by a computer to create an graphical representation of the part.
A significant trend with respect to CMMs is a process of measurement known as scanning in which a probe tip (ball) is in constant contact with the part being measured. Probe tips are preferably spherical in shape to maintain a constant distance to the center of the sphere regardless of the probe or work-piece (part) orientation. Eventually friction between the work-piece and the probe tip causes flat spots to form on the probe tip. This happens especially quickly during repetitive measuring tasks of hard materials such as diamond form tooling. The flat spots negatively impact the accuracy of the measurement and therefore necessitate that the probe tip be replaced.
U.S. Pat. No. 5,208,993 to Harding (May 1993) addresses at least some of the problems related to contact between the probe tip and the part being measured. Harding teaches a probe having a biasing mechanism that includes a spring. Because the biasing mechanism allows some degree of play, there may be relatively less wear of the probe tip. However, problems related to wear of the probe tip persisted even after employment of a biasing mechanism.
In addition to problems resulting from friction, there are problems related to the affixation of the probe tip to the probe stem. Affixing probe tips by adhesive has proven to be problematic, because the shape and composition of the probe tips make adhesion generally unreliable. Moreover, vibration and friction from the scanning process often cause such probe tips to separate from the probe stem. To counter this problem, a probe tip may be drilled and securely fastened to the probe stem using the cavity created by drilling. Known drilled tips used for scanning are often made of ruby or sapphire, but those balls are particularly prone to wear related problems and to picking up particulate matter during scanning.
There is a need for improved probe tips that have a relatively low coefficient of friction and are receptive to secure attachment to a probe stem.
The present invention provides a probe tip (ball) made of silicon nitride that has a drilled cavity sized and dimensioned to receive a probe stem.
A ball constructed out of Si3N4 would likely be less susceptible to formation of flat spots, less susceptible to accumulation of particles, and have a lower coefficient of friction. Such balls tend to produce a more accurate measurement for a longer period of time because of the reduced wear and the reduced susceptibility to accumulation. Additionally, a silicon nitride ball would likely last up to five times longer than a ruby ball, and be less likely to damage the scanned surface. Up to now, the possibility of using Si3N4 has been discounted, at least partially because it was not recognized that a ball made of Si3N4 could be securely attached to a probe stem for use in scanning applications. Such lack of recognition may have resulted from a perceived inability to effectively drill a silicon nitride ball.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components.